The exterior of Jenny Packham's Gospel Oak studio in north London is drably nondescript. Inside it's a hub of sparkle, fantasy - and industry. Across two floors gridded with high tables, teams from her 70-strong staff are cutting, beading, sewing and packing the feminine fantasy dresses that are Packham's forte. Outside her office - which to find you have to negotiate a scaffold being used by workmen to expand the premises - a fittings model and a team of clipboard-equipped scrutinisers are subjecting a rail of dresses to minute assessment.

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Where these dresses are bound, who knows? But each will become emblematic of a meaningful moment for some woman, somewhere. Packham specialises in big-moment dresses that pack a punch, and her business is expanding rapidly. Her thorniest problem, she concedes, is establishing what her métier should be called. "Evening wear? I hate using 'evening'. Occasion wear? No. People are starting to say 'social wear' now, and I quite like that."

When Packham and her partner, Matthew Anderson (they have two daughters, Georgia and Isabella), started their company 26 years ago, building what they have today was pretty much the plan. She says: "I have always loved to create pieces that are special, unique. It's very important to me that the dress means something to someone. I've no real interest in making something that's an everyday item."

The first time she showed her clothes, it was to a reception of profound indifference: not a single order. "We thought that was fine! And then we went and started work on the next collection." Today she is plotting the renovation of her London shop in Mount Street, opening a new one in Hong Kong, planning a trunk show in Los Angeles and her next catwalk show in New York. Not to mention dispatching dresses to the ever-widening portfolio of well-known women who have discovered that Packham is red carpet gold.

"I really enjoy the diversity of the people we dress," she says. "This morning we're sending out dresses to Lady Gaga and Helen Mirren. And I'm very excited because we just dressed Anna Gunn from Breaking Bad. I have become obsessed with that show, absolutely obsessed." From Dame Angelina Jolie to Kate Winslet via Lena Headey and Taylor Swift, the red carpeteers turn to Packham for dresses that do indeed merit that tediously overused fashion descriptive, "stunning". Behind her office chair is a chart imprinted with every available colour of Swarovski crystal. A stencil at its base commands: "Do not remove from Jenny's office."

The Duchess of Cambridge in Jenny Packham: in 2011, in 2012 and on the Antipodean tour in 2014. Photo: REX

Sparkle and colour and richness, corralled within ladylike, sometimes sumptuous silhouettes, are her keynotes. One client who has become particularly partial to Packham is the Duchess of Cambridge: on tour, at evening events and even when she emerged from St Mary's Hospital with George in the crook of her arm. "It's fabulous to dress the Duchess," says Packham before pausing. But discretion is paramount? "Exactly!"

Packham posits that her mother planted the fashion bug in her. "She was very creative, always drawing, painting, sculpting, and making her own clothes. I would love to know what might have happened for her if she'd had the opportunity to go to art college." This was in Southampton, where Packham would accompany her mother to fashion shows at Tyrrell & Green or Debenhams (for whom she now designs a diffusion line, No 1 Jenny Packham). "It was a very gentle, provincial experience."

Both Jenny and her brother Chris, now a television presenter, were keenly fashion-conscious teenagers. "He was a punk rocker, but the most pristine punk rocker you have ever seen." When he started presenting The Really Wild Show, Jenny, still at college, ran up his shirts. "They were quite crazy, peppered with parrots and penguins and so, so Eighties." Today, "he mostly wears Prada and Comme des Garçons. Yes, he wears Prada on Springwatch too."

Anna Gunn from Breaking Bad in a Packham design. Get the look: dress, £184, and earrings, £25, by No 1 Jenny Packham at Debenhams; debenhams.co.uk. Crepe blouse, £270, Marni; matchesfashion.com

Watching the Hollywood greats in her childhood, reckons Packham, sparked that urge to make dresses with oomph. She says: "When I design I think about the desire someone might have for it, and why. I want them to fall in love with it. Nobody is going to buy one of our dresses because it will do, or as something to hide away in their wardrobe and wear at some dimly undetermined point. They always have an event of some kind in mind. They want to walk in the room and for everybody to think how amazing they look. That's the job, really."

Packham started her bridal collection eight years ago. Unsurprisingly, it is very successful, for what bride doesn't want to wow the room? Although she furnishes women for the pleasure of self-display, Packham herself is wary of it. Her own dress code is luxurious, but minimal and restrained of aesthetic. She is sweetly nervous of being photographed, but endures it gamely. So does she shy away from scrutiny? "I don't like it all. I don't like that feeling of being looked at."

Which might be why she puts so much effort into making that feeling such a pleasure for others.

On her personal style
When I'm designing I absolutely love colour, texture, print, embroidery and shine. Anything that twinkles, I'm all over it. I'm completely uninterested in adorning myself. It's enough for me to be surrounded by all this. So I like to retain some neutrality in what I wear.

On brides
Wedding dressing is really fascinating because often brides want to wear something that is completely outside fashion. Although we are now starting to see a lot of influence from what people are wearing on the red carpet coming into the bridal dresses. We are putting dresses from our catwalk shows into our bridal collection, with more crystal.

On Swiss Army brides
Brides love detachable things. If they can take something off and turn it into something else they'll be keen. They want to be able to deconstruct their dress so that they can wear it at the ceremony and in the evening too, when they want to be able to really dance. And you can't dance in a train.